In conjunction with engine control and powertrain systems in transportation vehicles and body systems for controlling air conditioners, the contents of control become more complicated and more systems are necessary for control. Accordingly, the number of electronic control units (ECU) mounted is increasing every year. There is a tendency that the number of electronic components mounted inside is increasing. While a plurality of electronic components and parts of different height generate heat, heat-conductive materials are now requisite to efficiently conduct the heat to casings of die-cast aluminum.
Further, since it is recently required to mount more electronic components and parts within a limited space, their mounting environment (e.g., ambient temperature, humidity, angle, and thickness) is more diversified. In the case of engine ECU, for example, there are more chances of installing electronic components and parts vertically in the engine room. As such, there are more chances of positioning heat-conductive materials vertically in the site where both vibration and high temperature are applied.
While heat-conductive materials are used in such environments, several proposals are made to prevent the heat-conductive material between the heat source and the cooling member from sagging and falling down, for example, use of heat-conductive silicone adhesive materials, heat-conductive potting materials, and room temperature-curable heat-conductive silicone rubber compositions (see JP-A H08-208993, JP-A S61-157569, JP-A 2004-352947, JP 3543663, and JP 4255287: Patent Documents 1 to 5).
However, since all these materials assume a high hardness and bond to substrates, there are drawbacks that they are least re-workable and apply stresses to the heat-generating electronic components. Also, since the heat-conductive material cannot withstand the repeated stresses due to thermal strain, it may separate from the heat-generating component or crack, leading to a rapid increase of thermal resistance.
Under the circumstances, an addition one-part heat-conductive material was discovered (JP-A 2003-301189: Patent Document 6), which has previously undergone heat crosslinking reaction to a high viscosity (to maintain flexibility) during its preparation so that the material is sag-controlled. Since this material is quite flexible despite a high viscosity, its impact of applying stress to the electronic component is weaker than high-hardness materials. Since this material freely deforms and conforms to an irregular surface, it is suitable to apply to electronic components of different height. However, there is the tradeoff as a matter of course, that is, the problem of difficult coating because of high viscosity.
Recently, a heat-conductive silicone composition having a lower viscosity than the addition one-part heat-conductive material was developed (JP-A 2009-286855: Patent Document 7). Its viscosity is still high. There is a desire to have a heat-conductive silicone composition having better workability and sag control.
The above problem is solved by an addition one-part heat-conductive silicone composition (JP-A 2002-327116: Patent Document 8). That is, this composition is readily dispensable prior to heat curing, has a certain degree of re-working even after heat curing, does not sag after curing, remains as a relatively flexible rubber even after curing, and thus plays the role of a stress relaxing agent. Nevertheless, this addition one-part heat-conductive silicone composition still has a problem to be solved. The problem is that as the addition one-part heat-conductive silicone composition is further reduced in viscosity, the composition becomes flowable so that it may spread over the electronic component immediately after dispensing, failing to establish a heat-dissipating passage if a substantial space is defined between the electronic component and the cooling member.